1. Field of the Invention
The field of the invention relates to microelectromechanical systems (MEMS) and organic light-emitting diode (OLED) devices, and more particularly, to methods and systems for packaging MEMS and OLED devices.
2. Description of Related Technology
Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. Interferometric modulators may have a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In some interferometric modulators, one plate has a stationary layer deposited on a substrate and the other plate has a metallic membrane separated from the stationary layer by an air gap. The position of one plate in relation to another changes the optical interference of light incident on the interferometric modulator.
Cost effective packaging solutions that satisfy a variety of performance requirements are desired. Such requirements include resistance to moisture, minimized outgassing, and use of materials with similar coefficients of thermal expansion. One element in packaging technologies is the seal between materials used to create the package. The seal can provide mechanical integrity to the package, as well as a barrier to moisture and other contaminants. Seals are used in MEMS devices such as interferometric modulators, whose mechanical switching elements are susceptible to stiction caused by moisture or contaminants. Use of epoxies in sealing technologies can be limited by excessive outgassing, which can deposit harmful organic material on sensitive elements. Ceramic and metal packages can be welded or fusion bonded to yield hermeticity, but at relatively high cost. A less expensive method uses silica loaded epoxies to bond various packaging materials, but high hermeticity is generally not achieved.
Laser-based sealing has been used to seal materials in packaging applications. Typically, laser light is focused at the perimeter of a device, melting one or more materials at the perimeter and forming a bond. Some techniques deposit an interlayer between two materials to be bonded. The laser can be focused on and melt the interlayer to create a seal between the two materials. Graduated melting occurs, such that the melting takes place at the interface of two materials, or at an interlayer between the two materials, with the melt front growing gradually.
Unfortunately, in conventional laser-based sealing, a significant volume of material is typically heated to form the bond. In order to heat the material, the laser also heats the surrounding material to such a degree that it can affect neighboring components.
The effectiveness of laser-based bonding techniques is also limited by the optical absorbance of the materials present in the device. For example, lens systems are commonly used to focus laser light closer to the interface to be bonded, but the practical application of this technique is limited with transparent or optically homogenous substrates. Such substrates often absorb a substantial amount of the incident laser light before it reaches the interface. If insufficient laser light reaches the interface, there is insufficient heat generation and melting may not occur.
One bonding method creates a global bond using direct surface covalent bonding between pristine surfaces under high vacuum and temperature conditions, but at very high costs. Another bonding method uses anodic bonding aided by high electrostatic fields and elevated temperatures. Excessively harsh packaging conditions, including high temperature, make this method impractical for many device packaging applications, however.
Microwaves have also been used to heat metal layers placed at the interface of two materials to be bonded. Further, resistive heating with electrical currents flowing through metal traces have also been used to heat solders locally. These techniques suffer many of the same drawbacks noted above, however, including disadvantageous heating of a significant volume of material above and below the bond line. The resistive heating method is also impractical for MEMS applications because it requires high current density electrical interconnects through the wafer, complicating the fabrication process.